That Linda Ronstadt would lose her powerful voice after a four-decade singing career is the ironic curse of Parkinson's disease. The neurological degenerative condition affects more than 1 million Americans, robbing them of speech, mobility and their cognitive abilities.

The 67-year-old Grammy-winning singer revealed this week that she struggled with symptoms of the disease for nearly eight years before getting her diagnosis just months ago.

Ronstadt, half-German, half-Mexican, covered pop and folk with both sexiness and soul in the heyday of her career in the 1970s and '80s. Perhaps her biggest claims to fame were relationships with both California Gov. Jerry Brown and Star Wars director George Lucas.

"I can't sing a note," she told AARP in a piece to be published next week. "I knew it was mechanical. I knew it had to do with the muscles, but I thought it might have also had something to do with the tick disease that I had. And it didn't occur to me to go to a neurologist."

Her celebrity status, as well as that of Michael J. Fox, who in 1991 was diagnosed with early-onset Parkinson's at the age of 30, shines a spotlight on a disease that is still poorly understood and where little progress has been made beyond treating for symptoms. There is no cure and the prognosis for decline varies individually.

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I personally know many people with Parkinson's who have great humor about their trials and tribulations in life. - Joyce Oberdorf, National Parkinson Foundation

Today, at 52, Fox is starring in a new television series that aired for the first time this week and makes fun of his disease. He stars as Mike Henry, a New York news anchor who quits after a Parkinson's diagnosis but goes back to work.

"I have challenges that come with Parkinson's but my experience is to deal with things through humor," he told People magazine.

Fox has said that his first symptom was a shaking pinky finger. Ronstadt said her hands shook, but she thought it was the result of shoulder surgery. Today, she walks with the aid of poles on uneven ground and uses a wheelchair when she travels.

An estimated 50,000 to 60,000 new cases of Parkinson's disease are diagnosed each year in the United States, according to the Miami-based National Parkinson Foundation. The average age for onset is 62, but about 10 percent of all patients, like Fox, are under 40.

Some have lived 20 years or more with the disease before getting a diagnosis.

By the time patients discover the classic shake, which happens when the finger or hand is at rest, the disease has progressed "significantly," according to the foundation's CEO, Joyce Oberdorf.

"Incidence increases as people age," she said. "The brain is more vulnerable."

The main symptom of Parkinson's disease is a deficiency in dopamine, the "feel-good" chemical in the brain that affects mood and movement.

Dopamine relays messages between the substantia nigra and other parts of the brain to control coordinated muscle movement. When approximately 60 to 80 percent of the dopamine-producing cells are damaged, the motor symptoms of Parkinson's disease appear. This process of impairment of brain cells is called neurodegeneration.

Some theories are that the earliest signs of the disease start in the olfactory bulb, affecting the sense of smell.

"The first signs can be very subtle," said Oberdorf. "You might notice things like your handwriting is dramatically smaller, or notice a lack of smell or you are not sleeping as well at night. Or you feel stiff or rigid."

Most often, a slight tremor in the hand or other body part, "shaking without any movement ... all by itself," sends a patient to the doctor, she said. "When we see the tremor, that is the tipping point."

The disease can be highly variable. Those with tremor see a slower progression of the disease than those with rigidity, which is accompanied by more cognitive problems.

The voice is affected because it is controlled with muscular tissue. Patients often have trouble swallowing and breathing, as well. Most often those with Parkinson's don't realize their voices are softer, Oberdorf said.

"I am a huge fan of Linda Ronstadt," she said. "There are voice therapists that help, but you have to keep practicing."

The singer's announcement can have a positive effect on research, according to Oberdorf.

"If a celebrity has the personal ability to use the media spotlight for greater good, that's great," she said.

The disease was named in 1817 by Dr. James Parkinson, who published, "An Essay on the Shaking Palsy," based on six cases he had observed in his London medical practice.

Researchers now say that most cases of Parkinson's disease are not genetic but "sporadic."

The latest thinking is that the disease is rooted in the function and degradation of mitochondria, the cells' power producers.

"Brain-wide death of dopamine is only one part of a very extensive disease," said Virginia Commonwealth University's Bennett.

Therapy is only symptomatic with drugs like levadopa to replace lost dopamine nerve cells and deep brain stimulation to silence overactive ones.

"Treatment for the underlying disease does not exist," he said.

But people who start young and eat a so-called Mediterranean diet, rich in fruits and vegetables, can "stave" off the disease, according to Bennett.

One Japanese study, comparing the healthy Asian diet, full of fish and mushrooms, to the typical meat and fat-heavy Western diet, showed a decreased risk of 40 percent.

"It's what you put in your grocery cart -- diet is a big deal," he said.

There has also been some research to suggest an association between Parkinson's and exposure to pesticides.

Exercise is also a key to reducing risk.

"I am the darling of all personal trainers," Bennett said. "I beat on to [patients] about lifestyle changes because the ones who do have so much better quality of life and it can have a major impact on progression of the disease."

As for Ronstadt and Fox, their openness about having Parkinson's disease can only help the cause of awareness and research, according to both Bennett and Oberdorf.

"I personally know many people with Parkinson's who have great humor about their trials and tribulations in life," said Oberdorf.

"Having said that, it certainly complicates your life and you need to take care of yourself and learn to live well with it," she said. "It is, particularly in the later stages, supremely difficult and difficult on family."